Traffic signals have been used for many years. In North America alone, it is estimated that there are over 330 thousand signalized intersections. For municipalities and other road authorities, installation and maintenance of traffic signals can be a major budget item.
One form of traffic signal uses an incandescent bulb that is threaded to a lamp socket (or base) installed in a reflector assembly within a back recess of the signal housing where the bulb accesses a power source. The light bulb's white light is transmitted through a translucent lens that is colored (usually solid red, yellow, or green). This dome-shaped lens is installed in the front window of the housing. It is held in place in the window by a set of (usually four) mounting brackets (or “lens clips”). The mounting brackets are substantially flat stamped pieces of metal (usually stainless steel or aluminum) that are, more-or-less permanently, screwed onto the front door of the traffic signal housing (usually one in each corner). The mounting bracket extends over the circumferential lip of the lens to hold the lens in place.
To change the light bulb, a technician opens the traffic signal housing, unthreads the burnt out light bulb from the socket, screws in a new light bulb, then closes the housing. This is a relatively simple procedure. However, to replace the lens a technician needs to unscrew each of the mounting brackets (trying not to drop or lose the tiny screws or the brackets themselves), remove the broken or damaged lens, then replace the lens with a new one, reattaching the mounting brackets by re-fastening the screws with a screwdriver (while holding the lens in place).
These replacements are typically done by a technician on-site. The technician is in an awkward position—raised up in a “bucket” of a crane truck. The buckets tend to sag and sway in high winds. The technician must work quickly and carefully. The traffic signal usually needs to be disabled to carry out this maintenance work, so time is of the essence. Since the signal is outdoors, the technician may also be working in less than ideal weather conditions or under cover of night. He may further be using gloves to do the replacement, adding to the difficulty. All of these factors make simple operations more difficult.
A recent development is the use of light emitting diodes (LED's) in traffic signals. Gradually, road authorities in various jurisdictions have been switching over from the conventional incandescent light-based traffic signals to LED-based traffic signals, which last longer and require less power than incandescent light bulbs. It is estimated that roughly 75% of the traffic signals in North America have been converted to LED. One way this change has been implemented has been by retrofitting signals designed for incandescent-type lamps to use an LED lamp kit.
LED lamp kits are structured differently than traditional incandescent lamps. Instead of having a separate lens, bulb, socket/reflector and power supply, the LED lamp kit is a self-contained unit. All of these parts are in one package. An array of LED bulbs is mounted on a circuit board. The circuit board is connected to a power supply. A colored or clear dome lens is attached over the circuit board. All of these parts are contained in a single sealed “kit”. One standard type of kit is a round disc-shaped kit (there are also square and rectangular kits). The diameter of the round kit is commonly 8 inches (200 mm), 12 inches (300 mm) or 16 inches (400 mm). To retrofit the incandescent housing, the socket and reflector are removed, the lens is removed, and the LED lamp kit is simply installed in place of the lens. The lamp kit is attached using the same screw-fastened mounting brackets that were formerly used to hold the lens in the incandescent model.
A problem with this retrofit is that the LED lamp kits must be replaced at fairly frequent intervals. Unlike the lenses of the incandescent model (which were infrequently —if ever—replaced after installation), the LED lamp kits are routinely swapped out for fresh kits approximately every 3-5 years. Therefore, the technician is confronted with the hassles of dealing with the screw-fastened mounting brackets much more frequently.
It would be desirable to provide a system whereby either lenses or full LED lamp kits can be easily replaced in a traffic signal without the need for detachable parts or tools, allowing the technician to carry out the replacement simply and effectively.